How Does Soap Work?

There’s a complex chemical in our homes that we use daily with little regard to what it is or how it works — soap. This substance seems simple. Just rub it on your hands or pour it into a washing machine and instantly make something clean. But using soap can be important, for example, if you want to prevent boils.

The Basics of Soap

Soap is the name we give any blend of sodium or potassium-based “salts” of fatty acids. When you think of the word “salt”, don’t think of the stuff on the kitchen table — in chemistry terms, a “salt” is a compound that forms when acids become chemically neutralized. These compounds are produced when fats undergo a process called “hydrolysis”. This process is known as saponification, or soap-making.

Each molecule of the soap on your kitchen counter or under the bathroom sink contains a long chain of hydrocarbons (known as the “tail” of the molecule) and a chunky”‘head” made of carboxyl acids. Look at the structure of a molecule of soap and you’ll be reminded of a tadpole.

How Does Soap Clean Things?

When it is introduced to water, the sodium or potassium ions of the soap float away freely, leaving behind an oppositely charged “head”. To explain how this works as a cleaning agent, think of putting soap into dirty water.

When you put soap into dirty water it is easy to see its emulsifying properties. We call soap an emulsifier because of its ability to disperse liquids into each other, especially for its ability to mix water with another liquid that normally wouldn’t mix well. You know how oil doesn’t mix well with water? The use of a soap will help suspend oil or dirt so it can be washed away with water.

Seems complicated, right?

More Chemistry

Remember that negatively-charged molecule we talked about? It turns out that the “head” of a soap molecule undergoes some big changes when it contacts water. The other part of the molecule (the hydrocarbon “tail”) does not change in the presence of water.

What happens next is pretty cool — the various “tails” of the soap molecules come together to form “micelles” which are like tiny spheres of negatively-charged ions, with hydrocarbons inside them. The bottom line is that these micelles are negatively-charged and push away from each other, allowing them to stay suspended in water.

Getting Rid of Grease and Oil

Certain compounds are insoluble in water — grease and oil are two of these that we combat with soap all the time.

Dirty oils (when mixed with soap) are broken up by the soapy micelles. Interestingly enough, this forms a different compound much like the original micelle that traps the dirty molecules in the center of the compound.This trapping of dirt and oil is what makes soap turn us squeaky clean — the oils and dirt can be easily washed away by plain water which would not normally be strong enough to get rid of the grease and oil.

The Dirty Side of Soap

Natural soaps are some the best cleansers we have, but these natural products come with a few downsides. Remember that natural soaps are “salts” of pretty mild acid compounds. This means that mineral acids are used to form natural soap from fat, and some of these fatty acids are not soluble in sodium or potassium and form “soap scum”.

This also explains why tap water (tap water is pretty much “acidic” everywhere in the US) is not a really effective medium for natural soap. Another form of water that doesn’t work well with natural soap is hard water, or water that naturally contains chemical compounds like calicum, iron, or magnesium. This is why it is hard to lather up in hard water.

The next time you dirty a plate and slide it into the dishwasher without a moment’s thought, think of the complex chemical reaction happening inside of soap. Without the interaction of soap and water, we’d all be a lot less healthy and a lot stinkier.

This is part of a series of blog posts we’re publishing about How Stuff Works. The other posts in this series include: